Electroluminescent (EL) devices comprise a phosphor sandwiched between two electrodes. The phosphor can be dispersed in a dielectric medium or have dielectric layers interposed between itself and the electrodes. The phosphor is such that it will luminesce when placed in an alternating electric field. At least one of the electrodes is usually transparent to the light emitted by the phosphor. The opposite electrode can be contiguous with the entire phosphor layer, in which case a sample light source is produced, or it can be in a segmented form, such as a numeric or alpha-numeric. Such devices are known in the art. This invention concerns the latter devices. It is also known, relative to these latter devices, to employ a layer of material therewith to enhance the contrast between lit and unlit portions thereof to improve viewing under conditions of high ambient light.
U.S. Pat. No. 3,560,784, for example, discloses materials for this contrast enhancing layer as comprising sulfides, selenides and sulfoselenides (and mixtures thereof) of arsenic.
U.S. Ser. No. 974,279, filed Dec. 29, 1978 now abandoned and assigned to the assignee of the present invention, discloses a similar layer comprised of a mixture of cadmium telluride and lead telluride.
These suggested materials, however, have problems associated with their use. The arsenic compounds do not provide a satisfactory dark color and they have been known to change color with use. While the contrast enhancing layers comprised of the tellurides provide good results, cadmium telluride is a toxic material which is not recommended for industrial use.